proposal: swarm: use log to search messages in conversations

This gives to clients the ability to perform search for messages
with several parameters (account's id, conversation's id, author,
period, max number).
(To discuss) This patch introduces the search API, and a signal
(MessagesFound) to return a result.

GitLab: jami-project#1382
Change-Id: Ibc4665449fa0da71a015d1d18d6d0d3209331d43